1. Field of the Invention
The present invention is in the field of semiconductor manufacturing and more specifically relates to an improvement that permits a robot to handle wet thin wafers more reliably.
2. The Problem
In a widely used process, electronic circuits are deposited, layer upon layer, on a substrate of insulative material. The substrate has the shape of a thin disc, called a wafer, which may typically be 200 millimeters in diameter and several hundred microns thick. During the manufacturing process, the wafers are transported from one work station to the next by a robot. The portion of the robot that comes in contact with the wafer is called an end effector. The type of end effector with which the present invention is concerned acquires a wafer by hovering over it and applying a vacuum, which draws the wafer against the underside of the end effector. Once acquired by the robot, the wafer may be transported to a different work station where the wafer is released by relieving the vacuum and/or applying a slight overpressure.
For dry wafers of any thickness, this technique for releasing wafers works reliably. However, a problem arises if the wafers are wet; for example, if they have been rinsed but not dried. With wet wafers, the liquid forms a film between the wafer and the undersurface of the end effector which acts to cause the wafer to adhere to the end effector. With thick wafers, the weight of the wafer usually overcomes the tendency to adhere, but for wafer thicknesses less than approximately 130 microns, the weight of the wafer is not sufficient to overcome the adhesion, and such wafers are not released reliably. To make matters worse, the thinner the wafer is, the less able it is to withstand any applied overpressure, so that approach is not viable.
In hope of finding a solution to this problem, the present inventor turned to the prior art.
3. The Prior Art
In U.S. Pat. No. 4,904,012 issued Feb. 27, 1990 to Nishiguchi et al. there is described an elastic member located on the wafer-contacting surface of an instrument for picking up and releasing wafers. The elastic member serves as a seal between the wafer and the instrument.
In U.S. Pat. No. 5,622,400 issued Apr. 22, 1997 to George, an apparatus is disclosed for solving the problem of acquiring very thin wafers that have warped into the shape of a potato chip and therefore are not dependably acquired by the end effector. George's solution is to use an air jet to push the wafer into sealing contact with the end effector, where it becomes held by the vacuum.
In U.S. Pat. No. 5,676,590 issued Oct. 14, 1997 to Hiraoka, there is described a polishing apparatus in which surface tension is used to hold a wafer to a pad while the wafer is being polished.
In U.S. Pat. No. 5,685,588 issued Nov. 11, 1997, Wong et al. describe a wafer pad, composed preferably of plastic or rubber, that supports the wafer from below. The pad is annular-shaped and vacuum is applied through the central opening to hold the wafer on the wafer chuck.
Although Wong et al. and Nishiguchi et al. describe a pad or cushion interposed between the wafer and the end effector, these pads are needed to provide a seal between the wafer and the end effector. None of the prior art patents known to the present inventor even suggests that a problem might exist when wet wafers are handled. Thus faced with a problem that apparently had not been addressed by prior art, the present inventor set out to find his own solution.